CA1089498A

CA1089498A - Si.sub.3n.sub.4 hot-pressed with mgo

Info

Publication number: CA1089498A
Application number: CA292,005A
Authority: CA
Inventors: Frederick F. Lange; Clarence A. Andersson
Original assignee: Westinghouse Electric Corp
Current assignee: CBS Corp
Priority date: 1976-12-29
Filing date: 1977-11-29
Publication date: 1980-11-11
Also published as: AR215153Q; GB1597480A; US4099979A

Abstract

IMPROVED Si3N4 HOT-PRESSED WITH MgO

ABSTRACT OF THE DISCLOSURE
This invention relates generally to ceramic mate-rials formed from powder, and more particularly to hot-pressed structural materials comprising silicon nitride (Si3N4) wherein the oxygen content is controlled by main-taining the molar ratio of MgO and SiO2.

Description

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-s~rirltion ol' t},e ~rior Art _ __ __ , _ __ _ _ _ _ _ _ _ _ -~el'etOrOre it h2s been ~.nown in the art to hot-ess "i31~4 po~der ~ith the ~cldition of srnall amounts of ma-nesium o~.ide (MgO) as a densi~ication aid. See for ~;am?le ~ritish Patent Nos. 1,092,637 and No. 1,273,145.
Tne ~gO reacts with the silicon oxide (SiO2) surface layer on each Si3N4 particle to form a liquid at high temperatures nich aids in densifyin~ the Si3~4 particles by a solution precipitation mechanism. Upon cooling, the resultant den-sified mass consists of Si3N4 grains and a residual grain boundary phase. The addition of MgO therefore provides higher densities in the silicon nitride body than for the case where no MgO is employed. I~hile this MgO addition is beneficial from the densi~ication standpoint, we have deter-mined that the residual grain boundary phase including MgO
and S102 can become viscous at high temperatures allowing the Si3N~ grains to separate and slide under stress thus causing a degradation of the material's mechanical properties.

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SUMMARY OF THE IN~ENTION
In accordance with our invention high purity Si3N4 powder is used as a starting material and the MgO/SiO2 molar ratlo is controlled between 3 and 5 and the MgO content is controlled at less than 6% by weight. The hot-pressed, densified silicon nitride material of our invention provides ;~
a two-fold increase in strength at 1400C relative to com~
mercial grade hot-pressed Si3N4 and about 3 to 4 orders of magnitude decrease in creep strain rate behavior at elevated temperatures relative to the commercial grade materlal.
BRIEF DESCRIPTION OF THE DRAWINGS :~ :
In order to gain a better understanding of our ~ .:
invention reference is made to the drawings ln which~
Figure 1 is a graphical representation of the oxygen content of the powder wherein varying amounts of SiO2 have been added;
Figure 2 is a graphical representation of the ;.
variation in flexural strength as a function of the MgO/SiO2 molar ratio; `~
Figure 3 is a graphical representation of the .; ~ ~
load-deflection behavior of various materlals with differing . ;` :
MgO contents and MgO/SiO2 molar ratios;
Figure 4 is a graphical representation comparing `.:.
the creep behavior of the materials of our in~ention versus , ~
commercial grade silicon nitride; and .`~

Figure 5 is a graph similar to that of Figure 4.

¦ DESCRIPTION_OF THE PREFERRED EMBODIMENT

As noted above, tne grain boundary phase can become viscous at high t`emperatures allowing the Si3N4 grains to separate and slide under stress, causing a

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degradation of the material's mechanical properties. The temperature where the grain boundary phase becomes viscous and affects the mechanical properties is determined by its chemical composition. We have determined that certain impurities such as CaO that reside within the grain boundary phase lower the temperature where the degradation is first ~;~
observed.
Accordingly, the CaO content must be limited to about 200 ppm maximum. In accordance with our observations, we have determined that the high temperature mechanical properties of Si3N4 may be enhanced by fabricating a purer starting material. Hence, by utilizing a pure starting ;;
material the detrimental e~fects of the impurities on the grain boundary phase viscosity are minimized. The problem of grain boundary viscosity at high temperatures and under stress is still present, however, notwithstanding the ~act ;~
that purer starting materials are utilized. This problem we ;;~
discovered is related to the MgO content of the powder, and further, the MgO/SiO2 molar ratio.
High purity Si3N4 powder was produced by nitriding ;~ ~
Si powder with additions of 0.0, 1.0 and 3.0 wt.% SiO2. The - ;
phase content o~ the resulting powders was 83-93% ~C-Si3N4;
17-7% ~ -Si3N4 and ~ 1% Si as determined by X-ray dif~rac-tion analysis. The oxygen content of the representative powders was determined a~ter nitriding by the inert gas fusion, thermoconductivity method. Table 1 below reports ~i , ;
the impurity content o~ the Si3N4 powders produced.

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Spectrochemical Analyses of Westinghouse Si3N4 Starting powder (wt. %) Al 0. o8 ;: ;
Ag 0.001 B 0.001 Ca 0.016 ; -~
Cr 0.01 Fe 0.1 Mg 0.001 ~;
Mn 0.05 `~ Mo 0.003 Ni 0.01 Pb 0.01 ;~, ~
Sb 0.01 ;` : ;
Sn 0.01 ;;~`
Ti 0.01 Zn 0.01 Figure 1 graphically illustrates that the oxygen content for the different batches of Si3N4 powder produced is a function of the SiO2 added prior to nitriding. The oxygen content of the silicon powder nitrided was between ;
0.4 and 0.5 wt.%. It is reasonable to believe that the ~` oxygen content of the Si powder is due to a surface layer of ~;
~' SiO2 and therefore, as noted in Figure 1, the increase in ,~,, ~ ....
oxygen content is proportional to the SiO2 added prior to nitriding. The molar content of the SiO2 in the powder is , ranged between 1.7 and 6.7 mole percent.
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Si3N4 powders containing different MgO/SiO2 ratios were prepared by mixing between 2 and 8% by weight MgO with the Si3N4 powders containing different SiO2 con- ` ~
tent. In addition~ the MgO/S102 ratios o~ several powders ~ ~-were also varied by mixing both MgO and SiO2 into a powder with an oxygen content o~ 0.4 wt.% (equivalent to 0.75 wt.%
SiO2). Mixing and particle size reduction was performed by milling the powder with methanol in polyethylene bottles using tungsten carbide cylindrical grinding media. Oxygen analysis before and after milling showed no changes that could not be accounted for by the MgO addition.
After stir-drying, the milled composite powders were hot-pressed in a nitrogen atmosphere in graphite dies `~
with a stress of 28 MN/m2 at a temperature of 1750C between 1-4 hours to produce 5 cm diameter by 0.75 cm discs. Gra-phite dies with appropriate coatings were used in accordance ;
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with standard hot-pressing techniques. Densities were determined by water immersion. The densities of the hot~
pressed composite silicon nitride particles were between ~-3.20 grams per cc and 3.29 grams per cc. Bar speclmens .317 x .635 x 3.17 cm were sectioned and ground. Room tempera-ture flexural strength measurements were made at a crosshead ~ ~ ;
speed o~ 0.05 cm/min using a metal ~ixture (o.635 cm inner and 1.905 cm outer loading spans). Elevated temperature ~: .
measurements were performed in air at 1400C with a cross- ~;
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head speed of 0.005 cm/min using a hot-pressed S13N4 ~ix~
ture ~0.950 cm inner and 2.222 cm outer loading spans).
Figure 2 is a graphical representatlon of the flexural strength data at room temperature and at 1400C as a ~unction o~ the MgO/SiO2 ratio. The oxygen content o~ the - 46,834 ~t~ 9 ~

Si3N4 powder was used to calculate the SiO2 content and thus the MgO/SiO2 molar ratio. At 1400C the mean strength increased from 170 MN/M2 at low MgO/SiO2 ratios to 415 MN/M2 at an MgO/SiO2 ratio equal to 3. Therea~ter, the flexural strength decreased to 345 MN/M2 at higher MgOfSiO2 ratios o~
about 9. Fig. 2 indicates that where the MgO/SiO2 ratio was lowered by these additions of SiO2 to ratios of 1 and 2 produced low elevated temperature strength materials were ;~
produced. Without the additional SiO2 the same Si3N4 powders had greater MgO/SiO2 ratios and correspondingly higher strengths at 1400C.
Referring now to Figure 3, the load-deflection curve for selected specimens with different M~0/SiO2 ratios are depicted. As can be noted in Figure 3, less non-elastic deformation occurs at MgO/SiO2 ratios greater than 3.

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' The flexural strength of commercial hot-pressed ` -~
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~ Si3N4 is about 25,000-35,000 psi at 1400C compared to ~ `~

; between 45,000 and 70,000 psi ~or the material of our inven~
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tion with an MgO/SiO2 molar ratio o~ between 3 and 4. It is noted therefore that the material of our invention provides about a two-fold increase in flexural strength at 1400C
relative to the commercial Si3N4. Room temperature strengths ` ~ ;~
are similar for both materials.
In addition to the improved flexural strength of `
our materials, they also exhibit enhanced resistance to `;-creep at elevated temperature. The creep behavior of the materiais of our invention and that of the commercial Si3N
' material is illustrated in Figure 4 at 2550F and in Figure i 5 at 2300F. The material tested in Figure 4 was under a ;~
r; 30 stress of 15000 psi while the material of Figure 5 was under "~ ?~
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a stress of 30000 psi. As shown in the drawlngs, the creep resistance of the materials of this invention with an MgO/SiO2 ratio of 3 is approximately 3 to 4 orders of mag~
nitude better than the commercial Si3N4 material tested.
By malntaining the MgO/SiO2 molar ratio between ~-5 and more preferably between 3 and 4 and by malntaining the MgO content below 6 wt.% the mechanical property degradation of the material is increased by about 350F relatlve to the commercial grade Si3N4 materials. This incrsase in opera- ;~
ting temperature is signlricant for high temperature struc-tural materlals such as those employed ln gas turbine appli- ;
cations. The materials of this invention therefore are particularly suited for such components, for example, tur-bine blades and vanes.

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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A structural material suitable for use at elevated temperatures consisting essentially of silicon nitride, silicon dioxide and magnesium oxide, the molar ratio of MgO to SiO2 being within the range between about 3 and about 5, the MgO content not exceeding 6% by weight and the CaO content not exceeding 200 ppm, and the silicon nitride phases are between 83% and 93% alpha phase and between 7%
and 17% beta phase and silicon is ? 1%.

2. Hot pressed reaction sintered silicon nitride suitable for use at elevated temperatures consisting essenti-ally of MgO, SiO2, Si, CaO and Si3N4 with the MgO content not exceeding 6% by weight, the molar ratio of MgO to SiO2 being within the range between about 3 and about 5, a silicon content ? 1% and the balance silicon nitride, said sintered nitride having a CaO content not exceeding 200 ppm, and the silicon nitride phases are between 83% and 93%
alpha phase and between 7% and 17% beta phase.

3. The composition of claim 2 in which the MgO to SiO2 ratio is within the range between about 3 and about 4.